The invention relates to fuel tanks, and more particularly to fuel tank vents which permit a bi-directional equalization of pressure between the interior and exterior of the tank.
Fuels contained in tanks (and their vapors) undergo substantial thermal expansion. Because of the explosive nature of fuels, it is standard practice to provide some means of preventing an excessive buildup of pressure inside fuel tanks. Continuous, open venting of a fuel tank (such as a tank found on a lawn mower) is undesirable due to the potential for loss of fuel by evaporation, the discharge of noxious, explosive, and environmentally harmful vapors into the atmosphere, and the danger of spillage. Therefore, check valves are typically used to allow venting only above a threshold pressure.
A related problem is that as fuel is used (or as cooling occurs after venting) a partial vacuum can be created in the tank. If the vacuum becomes too great, engine performance can be affected. Thus, a need also exists to bleed air back into the tank at certain times.
Bi-directional fuel tank vents were therefore developed. These valves allow for substantial equalization of pressure in both directions during abnormal conditions, while otherwise maintaining the tank sealed. In operation, the bi-directional vent is normally closed, preventing escape of the vapors inside the tank. If the pressure inside the tank rises, for example due to thermal expansion, then the bi-directional vent allows just enough gas out of the tank to reduce pressure to the threshold differential. If the pressure inside the tank falls, then air is admitted into the tank to reduce the differential to the threshold differential. In both directions, a small pressure differential is required to initiate the venting action. Below that pressure differential, the vent is closed and the tank remains sealed.
While prior bi-directional vents are generally satisfactory, they typically require relatively expensive and complex seals. One common type of prior bi-directional vent incorporates a duck bill type seal for providing venting in one direction only, usually into the tank. In that case a second check valve, usually formed around the rim of the vent, is used to provide venting in the other direction. In the case of the duck bill seal, the seal lips may become jammed by debris, thereby compromising their effectiveness. Further, some such vents are susceptible to leakage when the tank: is tipped over (e.g. a snow blower falls on its side).
Thus, a need has existed for a bi-directional valve which uses a seal that is inexpensive to produce, which has a self-cleaning mechanism, and which is resistant to fuel leakage when the tank is tipped over accidently.